If you have a bypass on the hot water heater you will see this. Cold water enters tank at bottom and hot out the top. If you have a bypass on the water heater you will see a water line connecting top to bottom line. You will also see two or three valves. One at bottom between top to bottom line and tank. One on line between top and bottom lines. And possibly one at top line between tank and top to bottom line. If you do not see this valve there will be a check valve on top line hot outlet.
Close lower valve, and top valve if you have one. Open valve on line between top and bottom lines. Then RV antifreeze will bypass tank. Take out drain plug at outside and open pressure release to drain tank.
NOTE; On some newer RV's the bypass valves may be where the 'T' fitting would be at top and bottom lines. I think that you just turn valve in direction that you want antifreeze to go.
I do not recommend compressed air unless you have a CLEAN air source, as there is rusty water and oil in compressor tanks that will get in your water lines.